In the art of developing diazotype copy sheets with gaseous ammonia various approaches have been used to provide the ammonia gas and to secure its effective and uniform application to the copy sheet surface. Some of these are rather complex and expensive, and others, while simpler in concept and thus less costly, are not quite so efficient either in development speed or uniformity of development results. This trade-off between cost and effectiveness has always been a problem, and there is in the art an ongoing experimental effort seeking to reach a simpler constuction without impairing development efficiency.
One method of development involves the introduction of anhydrous ammonia gas into a developing chamber through which the copy sheet is passing, and one arrangement for this purpose is illustrated in U.S. patent to Bennett U.S. Pat. No. 3,900,862. In this patent one pump is used to slightly pressurize a reservoir of aqueous ammonia by pumping a gaseous medium into the same below the level of the liquid therein. This causes the anhydrous ammonia gas above the liquid to move out of the reservoir through a supply duct to the development chamber which is connected by an exhaust duct with the pump intake to complete the circuit. A second exhaust duct leads to a second pump via a control or "vacuum surge" chamber, and thence through a neutralizing bath to the atmosphere. The control chamber includes a relief valve designed to prevent the second pump from drawing anything greater than a constant predetermined vacuum in the development chamber.